The present invention relates generally to improvements in paper corrugating machinery. More particularly, the invention relates to a corrugating method and an improved flute profile for meshing corrugating rolls.
The basic assembly of corrugated paperboard comprises a sinusoidly undulating web that is adhesively laminated between two, oppositely facing flat webs. The undulating web is characterized as the corrugated medium or, alternatively, simply as the "medium". The two flat facing webs are characterized as liners. The undulations of the medium are called flutes. Respective to one side of the corrugated medium, the flutes are profiled between tips and valleys, alternately. The tip of one side of the medium constitutes a flute valley for the other side of the medium.
The corrugated medium is formed in a meshing nip between two corrugating rolls. These corrugating rolls are a matched pair of cylinders rotating about respective axes. Either or both may be directly driven. The surfaces of these cylinders have a fluted profile in a manner similar to meshing gears. An undeformed paper web is routed into the meshing nip between two rotating corrugating rolls. In the vicinity of the axial plane respective to these two rolls, the web fibers are deformed by a compressive or mildly crushing nip between the flute tip of one corrugating roll and the flute valley of the cooperating roll. This compression along the flute tip gives the web a permanently arced contour.
As the fluted medium emerges from the contour forming nip, it is held tightly to the fluted surfaces of one of the rolls by pressure, vacuum or by crescent shaped guide fences called "fingers" as it continues around an additional rotational arc about the roll axis into nipped convergence with a first liner web between the smooth surface of a pressure roll or belt and the passing flute tips of the corrugated medium. In route to the convergent nip, adhesive is applied to the medium flute tips. The line of contact along the axial plane between the pressure roll or belt and the corrugating roll provides significant compressive contact force between the liner web and the corrugated medium tips to induce an initial setting of an adhesive bond between them. The laminated product resulting from this compressive contact is characterized as single-face board and the aforedescribed machinery for producing it is characterized as the single-facer.
With respect to the compressive pressure applied to the corrugated medium web along the axial plane between the two corrugating rolls, the meshing flute tip and valley profiles are generally configured as partial arcs of concentric circles. The arc of such a partial circle is generally in the order of 40.degree. to 60.degree.. The circle radius is determined by the flute size. The clearance between the flute tip surface and the cooperating flute valley surface is in the order of 20 to 40 percent less than the thickness of the medium web. As a term of art, a web thickness dimension is often characterized as "caliper". Hence, a mildly compressive crush of the medium or a decrease in caliper occurs along this circular arced nip space between a flute tip and valley.
The material substance of corrugating medium paperboard is usually quite abrasive. Normally, it is the product of a high yield pulping process controlled to remove only about half of the naturally present lignin. Such high yield pulp usually receives only minimal cleaning and dirt removal. Additionally, high yield medium pulp is often supplemented with the fiber rejects from other paper products and pulp additives consisting of drainage and formation aids which can include highly abrasive particles.
Since the meshing nip between flutes of a pair of corrugating rolls is not a true involute and occurs via co-rotating motion of rolls in compression, the compressive web contact between a meshing flute tip and valley necessarily includes a degree of wiping or sliding motion between the medium web and the merging and diverging flute surfaces. In combination with the nip compression force and the abrasiveness of the medium web, this sliding between adjacent meshing corrugating roll flute tips and valleys is a significant source of roll material loss due to abrasion. When the degree of roll material loss along the flute tips and valleys progresses to the point that unacceptable product is produced, the corrugating rolls must be replaced.
In the overall, a corrugating roll is a highly complex machine element of considerable capital value. The quantity of structural material lost by abrasion is extremely small, both in area and in mass. Hence, it is a standard practice to hard-face or plate the roll surfaces with a relatively thin layer of tungsten carbide or electroplated chrome. These extremely hard surface coatings extend the operational life of a roll by slowing the rate of wear. Moreover, hard-facing facilitates roll recycling by known reconditioning processes. When a plated roll wears out, the coating is chemically removed and the metal value recovered. The chemically stripped roll is then resurfaced as new.
Within the fiber compressing nip between mating flute tips and valleys, the wear rate is greatest along the axial plane of tip and valley symmetry. However, electrolytically applied metal coatings often do not distribute uniformly onto small radius convex and concave surfaces. Corrugating roll flute valleys may receive a thinner protective coat of chrome or electrically deposited hard-face material along the axial plane than do the flute tips. Accordingly, the thickness of a flute valley coating is often the limiting factor in concluding a corrugating roll production cycle before reconditioning the roll surface.
It is, therefore, an object of the present invention to extend the operating life of a corrugating roll.
Another object of the present invention is to increase the electroplate deposition thickness along corrugating roll flute valleys which can be sustained during use before a roll has to be taken off line and replated.
An additional object of the invention is to provide an improved corrugating roll nip profile.
A further object of the invention is to provide a new corrugating process that results in a reduced rate of machinery wear.
A still further object of the invention is a corrugating machine that provides a more consistent corrugated board caliper over the productive life cycle of the machine.
It is also an object of the invention to provide a corrugated board product with improved properties including, but not limited to, improved adherence between the corrugating medium and the adjacent liner or liners and improved compressive strength.